Originally Posted by 
Old Farang
				
			 
			 Back in the days of sailing ships when I started the most common way to convert DC voltage was to use a "rotary converter". Some were motor driven "generators", either AC or DC motors, direct coupled or belt driven, or if you were REALLY posh, a self contained motor / generator. 
Then all you young blokes got in on the act and developed electronic versions. So basically; DC input is taken, converted to AC internally just like a DC inverter, adjusted up or down depending on what is required, then converted back to DC output.